The present invention relates to an ISDN user-network interface system and, more particularly, to establishment of initial synchronization.
Interface between a network terminal equipment and a terminal equipment of the ISDN is defined by the CCITT Recommendation I.400 series, and the physical layer of the basic access interface is defined by Recommendation I.430. Recommendation I.430 defines the presence of a one-to-one connection and a one-to-n connection as operating modes and defines the following maximum operational distance and line configuration or arrangement models: (a) short passive bus of 100 to 200 meters (1:2 point-to-multipoint configuration), (b) extended passive bus of 500 to 1000 meters (1:2 point-to-multipoint configuration) and (c) point-to-point configuration of a maximum length of 1000 meters. With such line arrangement models, a transmission delay between a network terminal equipment and a terminal equipment, and a distortion of the signal waveform can be held within values defined by Recommendation I.430. Since the line arrangement models are defined to satisfy electrical characteristics between a terminal equipment and a network terminal equipment, however, it is also possible to employ other line configurations, if they satisfy the electrical characteristics.
Moreover, Recommendation I.430 defines access control described below in a case where a plurality of terminal equipments concurrently access an up-line D channel to a network terminal equipment in an operating mode in which a plurality of terminal equipments are connected to one bus. That is, only one of the terminal equipments is allowed to correctly complete transmission of its information, the other terminals once stop their access to the D channel and, when the channel becomes idle, try to access it, and ultimately all the terminal equipments complete transmission of their information in a sequential order. To effect such access control for the D channel, the network terminal equipment, upon receiving a signal frame (hereinafter refered as "up-signal frame") on an up line from a terminal equipment, copies D-channel bits in the up-signal frame into echo channel bits in a signal frame (hereinafter refered as "down-signal frame") which is transmitted on a down line to the terminal equipment next. Each terminal equipment observes the echo bits in its received down-signal frame and compares the D-channel bits in the transmitted signal frame with the echo channel bits in the received signal frame, thereby deciding the state of the up-line D channel for collision detection.
In a case where connections other than those of the line arrangement models defined by Recommendation I.430, such as the extension of the transmission line and encoding, are carried out between the network terminal equipment and the terminal equipment in the prior art, electrical characteristics between the network terminal equipment and the terminal equipment sometimes may not satisfy the defined values. Of the electrical characteristics defined by Recommendation I.430, deterioration of the signal waveform can satisfy the defined value through use of the existing techniques such as a waveform shaping circuit and an amplifier. As regards the transmission delay between the network terminal equipment and the terminal equipment, however, the defined value cannot be satisfied even by the use of the existing techniques, Yet it is premised that the network terminal equipment and the terminal equipment are used in an environment in which they satisfy the defined values of Recommendation I. 430, and their normal operation is not guaranteed under the environment in which the defined values are not satisfied.
That is, when the transmission delay between the network terminal equipment and each terminal equipment exceeds the defined value of Recommendation I.430, two problems arises. One problem is that synchronization of the up-signal frame transmitted from the terminal equipment cannot be established in the network terminal equipment. In general, the network terminal equipment is produced on the assumption that it is used under an environmental condition where the electrical characteristics satisfy the defined values of Recommendation I.430; hence, when a delay is larger than the defined value, it cannot be said that synchronization of the up-signal frame transmitted from the terminal equipment can always be detected correctly.
The other problem is that when the round trip delay time between the network terminal equipment and the terminal equipment is longer than 250 microseconds, the down-signal frame which is received in the terminal equipment immediately after the transmission therefrom of the up-signal frame differs from the down-signal frame transmitted from the network terminal equipment in response to the up-signal frame, that is, the up-signal frame and the down-signal frame do not correctly correspond to each other in the terminal equipment. If an interval from a time when the network terminal equipment transmitted the down-signal frame to a time when it receives the up-signal frame which is transmitted from the terminal equipment in response to the down-signal frame is longer than 250 microseconds, then the network terminal equipment will transmit the next signal frame before it receives the up-signal frame. Then the down-signal frame which the terminal equipment receives immediately after transmitting the up-signal frame is a down-signal frame already transmitted from the network terminal equipment prior to the reception of the up-signal frame. Accordingly, the echo channel bits in the down-signal frame received by the terminal equipment have copied thereinto the D-channel bits in the signal frame transmitted before the immediately preceding signal frame. When the D-channel bits and the echo channel bits in the signal frame do not correspond to each other in the terminal equipment, D-channel access control cannot be effected.
Because of the two problems mentioned above, it is impossible, with the prior art, to perform an extension of the transmission line or encoding which causes the transmission delay between the network terminal equipment and each terminal equipment to exceed the defined value of Recommendation I.430.
Furthermore, since the line extension which causes the round trip delay to be larger than the defined value of Recommendation I.430 is impossible between the network terminal equipment and each terminal equipment, passive buses, each having connected thereto a terminal unit, cannot be connected to each other. Hence it is impossible to connect a plurality of passive buses to one network terminal equipment to simultaneously connect thereto eight or more terminal equipments.